Serena Williams: Would she have ruled in the 1990s?

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Photos:Tennis' calendar girl

Serena Williams in action during the 2016 US Open at the USTA Billie Jean King National Tennis Center in Flushing, New York. Here's a look at her many different fashion styles, on court and off.

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Serena attends the 2016 Vanity Fair Oscar Party Hosted By Graydon Carter at the Wallis Annenberg Center for the Performing Arts in Beverly Hills, California.

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Serena enjoyed an astonishing 2015 season -- winning the Australian Open, French Open and Wimbledon. She missed the China Open and WTA finals after revealing she needed time to recover from a grueling year.

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At the beginning – Serena sports a beaded hairstyle as she celebrates her first U.S. Open title -- and her first major -- back in 1999.

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Parisian style – Serena Williams poses on court after triumphing in the 2015 French Open final.

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In the pink – Serena blasts down a powerful serve on her way to another victory.

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Height of fashion – In 1999, Williams enrolled at the Art Institute of Fort Lauderdale in Florida to study fashion design. Here, she shows a collection of her designs at the 2012 Australian Open.

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If the cap fits... – Williams, the world's No. 1 player, serves during a training session ahead of the 2015 French Open in Paris.

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Traditional values – Williams celebrated her sixth Wimbledon title in 2015, resplendent in the All England Club's traditional all-white attire. It meant she held all four grand slam titles, going back to the 2014 U.S. Open -- her second "Serena Slam."

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The cat's whiskers – Serena sported a catsuit when she played Corina Morariu during the 2002 U.S. Open. That title was the third leg of her first non-calendar "Serena Slam," which she completed months later at the 2003 Australian Open.

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Making an entrance – Williams waves to the crowd as she enters stadium court before her match against Monica Niculescu of Romania at Indian Wells in 2015.

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Glowing performance – In a neon yellow outfit, Serena celebrates against Eleni Daniilidou of Greece during the 2012 Western & Southern Open in Mason, Ohio.

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Blue moment – Williams looks crestfallen as she reflects on a point that got away.

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Suitable occasion – A more formally-attired Serena celebrates one of her six victories at the U.S. Open.

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Black and white image – The Florida resident has triumphed at the US Open in New York six times.

Out of the shadows – Serena serves on a sunny day at Flushing Meadows, New York.

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Up in the air – Williams celebrates after winning the Madrid Open final against Maria Sharapova in 2013.

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Hair-raising – Serena sports a new style at the Australian Open in 2013.

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Story highlights

Serena Williams aiming for 22nd grand slam title at 2016 French Open

Williams closing in on record of Steffi Graf

At her best, Williams "impossible" to beat, says Arantxa Sanchez-Vicario

But four-time grand slam winner bemoans lack of competition

(CNN)Serena Williams has won 21 grand slams, is the undisputed No.1 women's tennis player on the planet and has dominated the sport for years -- so is winning one of four main tournaments easier than it used to be?

Only Steffi Graf has won more grand slams in the modern era than Williams with the German claiming 22 during her illustrious career, but one former tennis great believes that Serena wouldn't have had it all her own way in the 1990s.

"When Serena is at the top, it's impossible -- nobody can stop her," four-time grand slam winner Arantxa Sanchez-Vicario told CNN's Open Court show. "You can see the clear domination and I think, for me, and our era, that was impossible.

"The winner at every tournament had to beat three or four of the top players to win the tournament -- now this isn't happening."

Sanchez-Vicario competed in a golden era for women's tennis when the Spaniard and Graf competed against the likes of Monica Seles, Martina Navratilova, Gabriela Sabatini, Lindsay Davenport and Martina Hingis.

Now 44, she believes the dominance of the 34-year-old Serena and the lack of any real challengers has meant less drama in the women's game.

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Sanchez-Vicario, who won four of her seven meetings against Williams, including a last-16 victory at the 1998 French Open, says the American is only vulnerable when she drops to a level her rivals can reach.